1. Field of the Invention
The present invention relates to an optical component and a backlight unit using the optical component. In particular, the present invention relates to an optical sheet suitable for use in an image display device, such as a backlight unit of a liquid crystal display device, a projector screen, or a plasma display, and a backlight unit using the optical sheet.
2. Description of the Related Art
Conventionally, technology for controlling light has been applied in various fields and has become a useful part of our lives. In particular, screens that are large, as well as having high-definition, high-luminance, and high-quality, are being developed for image display devices.
Various optical components are implemented for the purpose of enhancing the luminance, widening the viewing angle, and improving the screen quality of the backlight unit of liquid crystal display devices, plasma displays, and projector screens, which are image display devices.
For example, an edge light system or a direct-type system is used in the backlight unit of liquid crystal display devices. In each system, an optical sheet is frequently used to improve the screen quality.
In other words, a prism sheet and a diffusion sheet are used as the optical components on a light-exiting surface of a light-guiding board in the edge-light-system backlight unit. The prism sheet enhances the luminance. The diffusion sheet widens the viewing angle and diminishes flaws. A reflective sheet is used as the optical component on the back surface of the light-exiting surface of the light-guiding board.
Similarly, in the direct-type-system backlight unit, the reflective sheet, the diffusion sheet and, in some cases, the prism sheet are used as the optical components.
Among such various optical components, the diffusion sheet plays the most effective role in enhancing the luminance, widening the viewing angle, and improving the screen quality.
In the conventional diffusion sheet, the surface is given a rough, crimped texture, beads are mixed in with the binder and applied to a plastic sheet, or light diffusion materials are embedded within the sheet.
In such a diffusion sheet, light transmittance and diffusion are inversely related. It cannot yet be said that an effective use of light has been sufficiently achieved.
FIG. 6 shows a relationship between diffusion angles and the luminance in a typical diffusion sheet in the backlight unit of a liquid crystal display device.
Although there are differences depending on the type of light-guiding board in the backlight unit, as shown in FIG. 6, when the haze of the diffusion sheet is increased, only the peak luminance decreases and the diffusion angle is not widened.
This phenomenon can be seen in many conventional diffusion sheets. In such diffusion sheets, even a small widening of the diffusion angle causes a significant amount of reduction in the luminance because light is isotropically diffused.
On the other hand, the diffusion angle, namely the viewing angle, is required to be narrowed to increase the luminance.
In light of such issues, a following diffusion sheet is disclosed in the invention described in Patent Literature 1 (Japanese Patent Unexamined Publication 2001-4813). In the diffusion sheet, a transparent base material includes a large amount of fine, spherical air bubbles. The shape of the air bubbles is a spheroid.
The diffusion sheet described in Patent Literature 1 aims to increase diffusion in a required direction by causing anisotropy in the diffusion direction. The object of the diffusion sheet is to suppress the reduction in luminance.
An anisotropic dispersion film having elliptical micropores is also disclosed in the invention described in Patent Literature 2 (Japanese Patent Unexamined Publication 2003-29030).
However, sufficient diffusion and anisotropy cannot be attained from the spheroidal air bubbles. There are also problems such as difficulty in controlling the dispersion of the air bubbles.
In addition, other than diffusion sheets, an optical component that is applied to the backlight in an edge-light-system backlight unit is disclosed in the invention described in Patent Literature 3 (Japanese Patent Unexamined Publication Heisei 2-17).
In the invention described in Patent Literature 3, a lens unit is formed on the light-exiting surface of the light-guiding board. A prism sheet, on to which a prism row is formed, is disposed so that the prism row faces the light-guiding board side.
According to an invention such as that described in Patent Literature 3, light is outputted in a diagonal direction by the lens unit formed on the light-guiding board. The prism sheet makes the outgoing light face the front, thereby acquiring a light that has high luminance.
However, in the invention described in Patent Literature 3, the lens unit formed on the light-guiding board has a regular structure. Therefore, the light is concentrated and, as a result, there is a disadvantage in that the viewing angle is narrowed.
Furthermore, in an invention described in Patent Literature 4 (Japanese Patent Unexamined Publication Heisei 4-9804), a formation of linear bumps on the light-guiding board is disclosed. The directions of a majority of the linear bumps are perpendicular to the incident surface.
According to the invention described in Patent Literature 4, the linear bumps have a lens effect. Therefore, the light spreading in a direction parallel to the light incident surface is concentrated, thereby acquiring high luminance.
However, in the invention described in Patent Literature 4, the linear bumps face the traveling direction of the light within the light-guiding board. Therefore, high light-exit efficiency is not developed and the acquired luminance is not very high.
Because a person gazes at the screen of the liquid crystal display device, even the slightest flaw becomes annoying. One flaw, among the flaws, that is considered to be particularly difficult to improve is the development of a luminescent line and a band-shaped light section and dark section near the incident end of the light-guiding board.
To solve this problem, for example, the following is disclosed in an invention described in Patent Literature 5 (Japanese Patent Unexamined Publication Heisei 5-88168). A process to roughen the surface of the incident surface of the light-guiding board is performed, and the incident light is diffused. As a result, the development of the luminescent line and the band-shaped light section and dark section is reduced.
However, there is a problem with the invention described in Patent Literature 5 in that the vicinity of the incident end becomes too bright with foreign light.    [Patent Literature 1] Japanese Patent Unexamined Publication 2001-4813    [Patent Literature 2] Japanese Patent Unexamined Publication 2003-290301    [Patent Literature 3] Japanese Patent Unexamined Publication Heisei 2-17    [Patent Literature 4] Japanese Patent Unexamined Publication Heisei 4-9804    [Patent Literature 5] Japanese Patent Unexamined Publication Heisei 5-88168